1. Field of the Invention
The present invention relates to high pressure vessels and a method for their assembly.
2. Description of the Prior Art and Background Considerations
The fabrication of containers for extreme pressures was originated and developed by P. W. Bridgman where concentric prestressed belts of high strength alloys are assembled about a container wall to place the innermost layer thereof under compression. Conventionally, prestressing is achieved by heating and expanding the outer belts with respect to the inner layers of the container. Upon assembly and cooling, the belts contract upon the inner layers. Such thermal expansion is limited to small dimensional changes, and does not easily generate large dimensional changes, e.g., approximately one percent. In addition, even if sufficient expansion were obtainable by thermal means, the required high temperatures would change the mechanical properties of the components. Therefore, it has been necessary to look to other means by which the necessary pre-stress can be obtained.
In the fabrication of certain types of optical fibers, extrusion at high pressure is employed. The cylinder surrounding the extrusion piston used in this application must be of great strength and be capable of containing the very high pressures developed during extrusion. In the present invention, sintered carbides, such as tungsten or titanium, were found suitable for the extrusion cylinder. These materials have the necessary strength and have no chemical or physical effect on the extruded fiber. However, these high strength materials are also brittle and have a tendency to crack under tension. Therefore, it is necessary to fashion the sintered carbide as a tubular insert and to enclose it under great compressive pre-stress within a sleeve of hardened tool or tempered alloy steel so that, even during the application of the high extrusion pressures therein, there would still be some remaining compressive force exerted upon the carbide insert. To assemble the insert in the sleeve for the reasons described above, thermal expansion and contraction was insufficient to generate the required large dimensional changes of about one percent. Also, as stated above, any heating necessary to obtain a thermal expansion would destroy the temper of the components.
Therefore, the titanium carbide die was inserted into the hardened tool steel sleeve first by concentrically tapering the mating surfaces of the insert and the sleeve with a slight oversizing of the insert with respect to the sleeve, and then by axially compressing the insert into the sleeve. Such a method of assembly, however, gave rise to a dual, contradictory problem in that low friction during assembly, and high friction after assembly for retention of the insert within the sleeve were required. For example, a titanium carbide insert lubricated with a high pressure lubricant and retained in a sleeve, was expelled with explosive force after release of the assembly pressure, even though the taper angle of the mating surfaces was only 1.2.degree.. Yet, if no lubricant were used, the assembly process would have caused galling, resulting in a scored interface between the insert and the sleeve with unknown and undesired mechanical properties.